Liquid toner images are developed by varying the density of pigmented solids in a developer material on a latent image bearing surface in accordance with an imaged pattern. The variations in density are produced by the corresponding pattern of an electric field extending outward from the latent image bearing surface, which is configured by the different latent image and background voltages on the latent image bearing surface and a voltage on a developer plate or roller.
In general, developed liquid toner images are neither solid nor homogeneous. Typically, a liquid toner developer contains about 1.5% to 2% solids and a developed image contains about 15%-25% solids. The developed image has a higher density closer to the latent image bearing surface and a "fluffy", i.e. loosely bound, region furthest away from the latent image bearing surface.
In order to improve transfer of a clean developed image from the latent image bearing surface to a substrate it is most desirable to ensure that, before transfer, the pigmented solids adjacent background regions are substantially removed and the density of pigmented solids in the developed image is increased, thus compacting or rigidizing the developed image. The compacting or rigidizing of the developed image increases the image viscosity and enhances the ability of the image to maintain its integrity under the stresses encountered during image transfer. It is also desirable that excess liquid be removed from the latent image bearing surface before transfer.
It is known in the prior art, as described in U.S. Pat. No. 3,955,533, to employ a reverse roller spaced about 50 microns from the latent image bearing surface to shear off the carrier liquid and pigmented solids in the region beyond the outer edge of the image and thus leave relatively clean areas above the background.
The technique of removing carrier liquid is known generally as metering. An alternative metering technique, described in U.S. Pat. Nos. 3,767,300 and 3,741,643, employs an air knife, but has not been particularly successful due to sullying of the background as a result of turbulence and consequent mixing of the background inversion layer with the surface layer of the carrier liquid.
In U.S. Pat. No. 3,957,016, the use of a positive biased metering roller is proposed wherein the metering roller is maintained at a voltage intermediate the image and background voltages to clean the background while somewhat compacting the image.
In the prior art it is known to effect image transfer wherein the image is brought into contact with a substrate backed by a charged roller. Unless the image is rigidized before it reaches the nip of the latent image bearing surface and the roller, image squash and flow may occur. This is particularly true if the substrate is a non-porous material, such as plastic.
In the prior art, liquid toner images are generally transferred to substrates by electrophoresis, whereby the charged image moves from the latent image bearing surface to the substrate through the carrier liquid under the influence of an electric field produced by a high voltage, associated with the substrate, which is of opposite polarity to the charge on the image particles.
The voltage and thus the field strength available for electrophoretic transfer are limited by the danger of electrical breakdown which can occur at both the input and output edges of the nip, due to the minimum of the Paschen curve being at about 8 microns. Thus, according to the Paschen curve, the voltage difference at the nip cannot exceed about 360 volts, if possible damage to the image and possible damage to the latent image bearing surface due to electrical breakdown are to be avoided.
Electrophoretic compaction of images prior to transfer thereof is described in U.S. Pat. No. 4,286,039 which shows a metering roller followed by a negatively biased squeegee roller. The squeegee roller is operative both for compacting the image and for removing excess liquid. The voltage that can be applied to the squeegee roller is also limited by the danger of electrical breakdown. The breakdown problem is least serious at the input to the squeegee roller since the meniscus present there acts to increase the minimum effective air gap. In the image areas, the breakdown problem is more severe since the fields produced by the squeegee roller and by the latent image bearing surface add. The problem is most severe at the exit edge of the squeegee roller at which a meniscus is substantially not present.
In U.S. Pat. No. 4,684,238 an unmetered image is initially transferred to an intermediate transfer member and is then metered by a metering roller having a voltage opposite to the charge on the toner particles making up the image. No discussion of the problem of electrical breakdown is presented.